Depletion of multiple nutrients in the Earth’s oceans could be simultaneously limiting the abundance of microscopic organisms that play a vital role in removing carbon dioxide (CO2) from the Earth’s atmosphere, according to scientists behind a new study.

An international team of researchers, including a scientist from the University of Southampton, has discovered that the growth of single-celled microbes called phytoplankton can be simultaneously restricted by the availability of multiple nutrients, rather than just any one of them.

Measurements of nutrient concentrations in the ocean surface, where these single-cell photosynthesising organisms live, typically show widespread depletion of many of these elements, which makes the findings – published in the journal Nature – of potential widespread importance for understanding the oceanic carbon cycle.

As well as helping to control atmospheric CO2 (one of the primary greenhouse gases), phytoplankton are a vital food source for larger marine life forms such as fish and whales.

The study, led by GEOMAR Helmholtz Centre for Ocean Research, in Kiel, Germany, is based on the results of an expedition conducted as part of the International GEOTRACES Programme on the German research vessel METEOR in the South Atlantic, off the south-west African coast.

A team of scientists performed experiments at various locations along an ocean route covering thousands of kilometres.

Adding three potentially limiting nutrients – nitrogen, iron and cobalt – in all possible combinations, then tracking the growth response of the phytoplankton community, revealed ‘co-limitation’ of growth (i.e. that the availability of multiple nutrients simultaneously was a limiting factor on growth) in the majority of experiments. In some cases, addition of all three nutrients was needed to maximise growth.

Nutrient co-limitation has been proposed by scientists before, but this study has proven it experimentally, over large oceanographic extents, for the first time.

Professor Mark Moore, from the University of Southampton, is one of the authors of the study. He said: “Phytoplankton productivity is a vital part of the health of marine food webs and the Earth’s carbon cycle. These experiments suggest that we may have to revise our understanding of how this system responds to both natural and anthropogenic changes in nutrient supplies to the ocean.

“Our long-standing theoretical understanding of nutrient limitation is based on systems switching between single limiting nutrients, so such widespread evidence for co-limitation is surprising, but it is actually beginning to appear as if the more we look for co-limitation, the more we find.”

The study also demonstrated marked reductions in the diversity of the phytoplankton types present following sequential addition of limiting nutrients.

Professor Moore added: “There are some intriguing hints here that the diversity of marine microbes may play a key role in enabling co-limitation to develop. However, fully pulling apart the underlying mechanisms and subsequently representing these in global ocean models remains a challenge.”

The study also involved the University of Liverpool and Dalhousie University in Halifax, Canada.

The work was funded by a Marie Skłodowska-Curie Postdoctoral European Fellowship, the European Commission and the DFG.